.\" Copyright (C) Markus Kuhn, 1996
.\" and Copyright (C) Linux Foundation, 2008, written by Michael Kerrisk
.\"     <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: GPL-2.0-or-later
.\"
.\" 1996-04-10  Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
.\"             First version written
.\" Modified, 2004-10-24, aeb
.\" 2008-06-24, mtk
.\"     Minor rewrites of some parts.
.\"     NOTES: describe case where clock_nanosleep() can be preferable.
.\"     NOTES: describe CLOCK_REALTIME versus CLOCK_NANOSLEEP
.\"     Replace crufty discussion of HZ with a pointer to time(7).
.TH nanosleep 2 2023-02-12 "Linux man-pages 6.03"
.SH NAME
nanosleep \- high-resolution sleep
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <time.h>
.PP
.BI "int nanosleep(const struct timespec *" req ,
.BI "              struct timespec *_Nullable " rem );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR nanosleep ():
.nf
    _POSIX_C_SOURCE >= 199309L
.fi
.SH DESCRIPTION
.BR nanosleep ()
suspends the execution of the calling thread
until either at least the time specified in
.I *req
has elapsed, or the delivery of a signal
that triggers the invocation of a handler in the calling thread or
that terminates the process.
.PP
If the call is interrupted by a signal handler,
.BR nanosleep ()
returns \-1, sets
.I errno
to
.BR EINTR ,
and writes the remaining time into the structure pointed to by
.I rem
unless
.I rem
is NULL.
The value of
.I *rem
can then be used to call
.BR nanosleep ()
again and complete the specified pause (but see NOTES).
.PP
The
.BR timespec (3)
structure
is used to specify intervals of time with nanosecond precision.
.PP
The value of the nanoseconds field must be in the range [0, 999999999].
.PP
Compared to
.BR sleep (3)
and
.BR usleep (3),
.BR nanosleep ()
has the following advantages:
it provides a higher resolution for specifying the sleep interval;
POSIX.1 explicitly specifies that it
does not interact with signals;
and it makes the task of resuming a sleep that has been
interrupted by a signal handler easier.
.SH RETURN VALUE
On successfully sleeping for the requested interval,
.BR nanosleep ()
returns 0.
If the call is interrupted by a signal handler or encounters an error,
then it returns \-1, with
.I errno
set to indicate the error.
.SH ERRORS
.TP
.B EFAULT
Problem with copying information from user space.
.TP
.B EINTR
The pause has been interrupted by a signal that was
delivered to the thread (see
.BR signal (7)).
The remaining sleep time has been written
into
.I *rem
so that the thread can easily call
.BR nanosleep ()
again and continue with the pause.
.TP
.B EINVAL
The value in the
.I tv_nsec
field was not in the range [0, 999999999] or
.I tv_sec
was negative.
.SH STANDARDS
POSIX.1-2001, POSIX.1-2008.
.SH NOTES
If the interval specified in
.I req
is not an exact multiple of the granularity underlying clock (see
.BR time (7)),
then the interval will be rounded up to the next multiple.
Furthermore, after the sleep completes, there may still be a delay before
the CPU becomes free to once again execute the calling thread.
.PP
The fact that
.BR nanosleep ()
sleeps for a relative interval can be problematic if the call
is repeatedly restarted after being interrupted by signals,
since the time between the interruptions and restarts of the call
will lead to drift in the time when the sleep finally completes.
This problem can be avoided by using
.BR clock_nanosleep (2)
with an absolute time value.
.PP
POSIX.1 specifies that
.BR nanosleep ()
should measure time against the
.B CLOCK_REALTIME
clock.
However, Linux measures the time using the
.B CLOCK_MONOTONIC
clock.
.\" See also http://thread.gmane.org/gmane.linux.kernel/696854/
.\" Subject: nanosleep() uses CLOCK_MONOTONIC, should be CLOCK_REALTIME?
.\" Date: 2008-06-22 07:35:41 GMT
This probably does not matter, since the POSIX.1 specification for
.BR clock_settime (2)
says that discontinuous changes in
.B CLOCK_REALTIME
should not affect
.BR nanosleep ():
.RS
.PP
Setting the value of the
.B CLOCK_REALTIME
clock via
.BR clock_settime (2)
shall
have no effect on threads that are blocked waiting for a relative time
service based upon this clock, including the
.BR nanosleep ()
function; ...
Consequently, these time services shall expire when the requested relative
interval elapses, independently of the new or old value of the clock.
.RE
.SS Old behavior
In order to support applications requiring much more precise pauses
(e.g., in order to control some time-critical hardware),
.BR nanosleep ()
would handle pauses of up to 2 milliseconds by busy waiting with microsecond
precision when called from a thread scheduled under a real-time policy
like
.B SCHED_FIFO
or
.BR SCHED_RR .
This special extension was removed in Linux 2.5.39,
and is thus not available in Linux 2.6.0 and later kernels.
.SH BUGS
If a program that catches signals and uses
.BR nanosleep ()
receives signals at a very high rate,
then scheduling delays and rounding errors in the kernel's
calculation of the sleep interval and the returned
.I remain
value mean that the
.I remain
value may steadily
.I increase
on successive restarts of the
.BR nanosleep ()
call.
To avoid such problems, use
.BR clock_nanosleep (2)
with the
.B TIMER_ABSTIME
flag to sleep to an absolute deadline.
.PP
In Linux 2.4, if
.BR nanosleep ()
is stopped by a signal (e.g.,
.BR SIGTSTP ),
then the call fails with the error
.B EINTR
after the thread is resumed by a
.B SIGCONT
signal.
If the system call is subsequently restarted,
then the time that the thread spent in the stopped state is
.I not
counted against the sleep interval.
This problem is fixed in Linux 2.6.0 and later kernels.
.SH SEE ALSO
.BR clock_nanosleep (2),
.BR restart_syscall (2),
.BR sched_setscheduler (2),
.BR timer_create (2),
.BR sleep (3),
.BR timespec (3),
.BR usleep (3),
.BR time (7)
